Page 25. Note how there are valves into the intake that involve the fuel tank and evap control...perhaps at start up you are getting something pressurizing in the first few moments of start up. DOES IT do it irrespectiev of how full the tank is?

Also, there is an idle air control something... at start up the DME controls air and fuel as the engine warms and stabilizes. Snoop around the directory for the website above, maybe you'll find something. There may not be a valve per se, but there is some way air is controlled at start.